FIG. 1 of the accompanying drawings is a schematic diagram of a conventional fuel injection system 10 for an internal combustion engine.
The fuel injection system 10 comprises a plurality of fuel injectors 12. Each injector 12 is arranged to deliver an atomised spray of high-pressure fuel to a respective combustion chamber (not shown) of the engine. The injectors 12 receive fuel at high pressure from an accumulator volume or rail 14, by way of high-pressure supply lines 16. The rail 14 comprises a reservoir for high-pressure fuel.
Delivery of fuel from the injectors 12 is controlled by an electronic control unit 18. When a fuel injection from one of the injectors 12 is required, the electronic control unit 18 sends an actuation signal to the injector 12, which causes actuation of a delivery valve (not shown) of the injector 12.
Fuel is pumped to the rail 14 from a storage tank 20 by a fuel pump assembly 22. The fuel pump assembly 22 includes a low-pressure transfer pump 24, which serves to convey fuel from the tank 20 to the pump assembly 22, and a high-pressure pump 26 which elevates the pressure of the fuel to the injection pressure, typically of the order of 2000 bar. Fuel is conveyed from the tank 20 to the pump assembly 22 by way of a low-pressure fuel line 28, and from the pump assembly 22 to the rail by way of a high-pressure fuel line 30.
An inlet metering valve 32, under the control of the engine control unit 18, is provided between the transfer pump 24 and the high-pressure pump 26 of the pump assembly 22. The inlet metering valve 32 determines how much fuel reaches the high-pressure pump 26, for subsequent pressurisation and delivery to the rail 14.
The fuel pressure in the rail 14 is regulated to a target value by the electronic control unit 18 in the following way. The engine control unit 18 determines the fuel pressure in the rail by means of a rail pressure sensor 34. When the rail pressure is less than the target value, the engine control unit 18 opens the inlet metering valve 32 so that the high-pressure pump 26 delivers fuel at high pressure to the rail 14. When the rail pressure is more than the target value, the engine control unit 18 closes the inlet metering valve 32 so that the fuel pressure in the rail 14 can decay as fuel is delivered through the injectors 12.
In practice, the inlet metering valve 32 is configured to allow a variable flow from the transfer pump 24 to the high-pressure pump 26 within the range from fully-closed to fully-open, so as to permit accurate control of the rail pressure. In operation, the electronic control unit 18 selects the appropriate flow rate through the inlet metering valve 32 by adjusting the magnitude or other property of the signal supplied to an actuator of the inlet metering valve 32.
When the inlet metering valve 32 is fully open, the rate of increase of the rail pressure is maximised. To reduce the rate of increase of the rail pressure, the flow through the inlet metering valve 32 is reduced to throttle fuel flow to the high-pressure pump 26. In this way, accurate control of the pressure in the rail 14 can be achieved. For example, when pressurising the rail 14, the flow through the inlet metering valve 32 can be gradually reduced as the rail pressure approaches its target value so as to avoid the rail pressure overshooting the desired target value. Also, in steady-state engine operating conditions, the inlet metering valve 32 can be set to an appropriate level so that the fuel delivered to the high-pressure pump 26 equals the amount delivered to the injectors 12 plus any internal leakages, in order to maintain a steady fuel rail pressure.
One kind of inlet metering valve 32 known in the art is of the normally-closed type, in which an electrical signal must be supplied in order to allow fuel flow from the transfer pump 24 to the high-pressure pump 26 through the inlet metering valve 32. In the absence of an electrical signal from the engine control unit 18, the inlet metering valve 32 remains closed, so that no fuel reaches the high-pressure pump 26 and so there is no fuel delivery to the rail 14.
Accordingly, in the event of a system failure that causes no signal to be supplied to the inlet metering valve, an inlet metering valve 32 of the normally-closed type will cause the non-delivery of fuel to the rail 14, and hence the supply of fuel to the injectors 12 will cease. This will result in the engine stopping, or failing to start.
In some applications, for example in marine and industrial applications, stopping of the engine or its failure to start is particularly undesirable.
This problem can be addressed by the use of another kind of known inlet metering valve 32, of the normally-open type. In an arrangement with a normally-open inlet metering valve 32, an electrical signal must be supplied to the valve 32 in order to reduce or to completely stop the flow of fuel from the transfer pump 24 to the high-pressure pump 26. In the absence of an electrical signal from the engine control unit 18, the inlet metering valve 32 rests in its fully-open position and the fuel pressure in the rail 14 increases when the transfer pump 24 and high-pressure pump 26 are operational.
In this arrangement, a system failure that results in no signal being supplied to the inlet metering valve 32 will not cause the cessation of fuel delivery to the rail 14. Instead, because the inlet metering valve 32 will remain fully open, fuel will be continuously delivered to the rail 14 so that the engine can continue to operate.
It will be appreciated that, in this failure mode, the fuel pressure 14 in the rail can no longer be moderated by the inlet metering valve 32 and, when the rate of fuel injection through the injectors 12 is insufficient to relieve the fuel pressure, the rail pressure could rise to damaging levels. To avoid the risk of damage to the system as a result of overpressure in the rail 14. a pressure-limiting valve 36 is provided. The pressure-limiting valve 36 opens when the rail pressure exceeds a pre-determined threshold level, to provide a path for the flow of fuel from the rail 14 to the low-pressure tank 20, via a return line 38. In this way, the rise of fuel pressure in the system to an unacceptable level can be avoided. However, the addition of a pressure-limiting valve 36 and the associated fuel return line 38 undesirably increases the cost and complexity of the system.
DE10247436 discloses embodiments of normally open and normally closed metering valves. Against this background, it would be desirable to provide a less complex fuel injection system that allows an engine to continue to operate whilst preventing overpressurisation of the fuel in the system.